The invention concerns a method and an apparatus for monitoring fluid movements and seafloor subsidence/reservoir compaction in hydrocarbon-producing fields by repeated relative seafloor gravity and depth measurements. Monitoring of hydrocarbon reservoir changes (as saturation, pressure, and compaction) during production is traditionally done by well measurements integrated through dynamic reservoir modelling. Geophysical techniques for measuring changes between the wells has emerged as useful technology in recent years, particularly repeated seismic measurements. The benefits of such observations and improved understanding of the reservoir behaviour during production are many: among others to optimize production/reservoir management, optimizing drilling of infill wells and improving estimates of remaining reserves.
A new system comprising an instrument for use in seafloor gravity observations has been designed and deployed. The system is called xe2x80x9cRemotely Operated Vehicle Deep Ocean Gravimeterxe2x80x9d (ROVDOG). The aim of the project was to perform repeated measurements of gravity and pressure in an oilfield to monitor the development of the reservoir. The actual field in question is the Troll field in the North Sea. Because of the requirement for accurate location of the measurement points (each to within one cm of the previous observation) a gravimeter was required which could be handled by the arm of an ROV and placed atop sea floor benchmarks. Such an instrument has been designed around a Scintrex CG-3M land gravimeter. Motorized gimbals within a watertight pressure case are used to level the sensor. An assembly of 3 precise quartz pressure gauges 22 provides pressure measurements which can be transformed to depth information. The instrument may be operator-controlled via a serial datalink to the ROV. A view of the data stream for recording can be monitored. In one embodiment of the invention, the serial datalink is according to the RS-232 standard. In a test run of the system, the instrument was first deployed in the Troll field during June 1998. A total of 75 observations were made at 32 seafloor locations over a period of 120 hours. The repeatability figure of merit is 0.027 milliGal for the gravity measurement and 2 cm for pressure-derived heights.
Scope of Work
The Troll field licence partners decided in 1996 to try to perform measurements on the Troll field in order to montitor changes caused by the gas production and the influx of water from the aquifier in particular. Among different solutions comprising well monitoring and repeat seismic monitoring to perform such measurements, repeated gravity measurements on the sea floor above the field was proposed by the inventors. In an internal study in 1997 the changes of the gravity field were identified as comprising the following factors:
I) water influx in the gas reservoir;
II) seafloor subsidence; and
III) gas density reduction.
I) The expected increase in gravity caused by water influx.
II) The expected seafloor subsidence due to reservoir compaction. This seafloor subsidence will cause a change in the gravity field, as measured at the seafloor, being proportional to the subsidence, due to the vertical gradient of the Earth""s gravity field. Thus it is necessary to monitor the gravity changes or xe2x80x9cthe gravity signalxe2x80x9d to within a resolution of the gravity corresponding to an elevation difference of a few centimeters.
III) Gas density reduction will give a reduction of the mass density of the reservoir, causing a reduction of the gravity field, i.e. of opposite sign with respect to gas/water rise.
Fujimoto et al. in xe2x80x9cDevelopment of instruments for seafloor geodesyxe2x80x9d, Earth Planets Space, vol. 50, pp. 905-911, 1998, describes instruments for monitoring differential displacements across a fault zone in the seabed, and examines their resolutions through seafloor experiments at relatively short baselines. The horisontal differential displacement is measured by an acoustic ranging system using a linear pulse compression technique being able to measure distances on the order of 1 km between markers with an accuracy of 1 cm. The leveling or vertical displacement monitoring of the seabed is planned to use an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. Fujimoto et al describes how ocean bottom pressure measurements can be used in two ways to detect vertical movements of the seafloor. An ocean bottom pressure array acts as a monitoring system of relative vertical movements. Variations of atmospheric pressure are mostly compensated at the sea surface. By simulating pressure and gravity one can discriminate between a pressure change due to vertical seafloor displacements and a pressure change due to vertical sea surface displacements:
Consider the seafloor rising by 1 cm. The pressure value will decrease by 1 cm of water column. Gravity will decrease by 2.2 microGal (xe2x88x923.068 microGal due to height change and +0.864 microGal due to reduced gravitational attraction of the global sea water).
Next, Consider the sea surface lowering 1 cm. The pressure in this case will also decrease by 1 cm of water column. The gravity in this case will increase by 0.432 microGal due to the reduced gravitational attraction of the local seawater.
In both of the above mentioned cases, pressure monitored at the seabed decreases, but the gravity changes differently. If measurements are performed with high accuracy, simultaneous measurements of pressure and gravity can discriminate between the two effects: sea surface level change and seabed level change.
Fujimoto et al. do not propose any method for monitoring changing parameters representing density and/or mass distribution of an underground sub-sea reservoir by means of gravimetric measurements with a gravity sensor on the sea-bed. Fujimoto proposes conducting series of relative gravimetric measurements with a gravity sensor and relative depth measurements with a depth sensor on survey stations arranged on a benchmark having fixed vertical position relative to the local sea-bed in a survey area over a suspected fault zone, the gravimetric measurements being relative to gravimetric measurements and depth measurements taken on a reference station on land. Fujimoto proposes correcting the relative gravimetric measurements for the corresponding relative depth measurements, producing corrected relative gravity values. The corrected gravimetric values are then used for interpreting seabed vertical motions, and not used for comparison between series of observed corrected gravimetric values with later series of observed corrected gravity values and interpretion of a difference of corrected gravimetric values in terms of a change of parameters representing density and/or mass displacement in the underground sub-sea reservoir. Although seabed subsidence monitoring over the reservoir zone is one major issue of the present invention, the gravity change represented by seabed subsidence is noise with respect to detecting gravity changes due to mass movements and density change in the reservoir. Thus, in the present invention, the gravity measurement due to seabed subsidence (or rise) must, in addition to tidal and drift corrections, be corrected for by corresponding water column pressure changes at the seabed.
Presentation of Relevant Known Art
Gravity monitoring has previously been applied in exploiting hydrothermal energy (Allis, R. G, and Hunt, T. M., Geophysics 51, pp 1647-1660, 1986: Analysis of exploitation-induced gravity changes at Wairakei geothermal field.; San Andres, R. B, and Pedersen, J. R., Geothermics 22, pp 395-422, 1993, Monitoring the Bulalo geothermal reservoir, Philippines, using precision gravity data.), and in volcanology (Rymer, H. And Brown, G. C., J. Volcanol. Geotherm. Res. 27, pp. 229-254, 1986, Gravity fields and the interpretation of volcanic structures, geological discrimination and temporal evolution.). Recently, efforts on measuring gravity differences above hydrocarbon fields on land have been reported (Van Gelderen, M., Haagmans, M. R., and Bilker, M., Geophysical Prospecting 47, pp. 979-993, 1996, Gravity changes and natural gas extraction in Groningen). For offshore fields, gravity monitoring has been initiated in a case known to us (Hare, J., Ferguson, J. F., Aiken, C. L. V., and Brady, J. L., 1999, The 4-D microgravity method for waterflood surveillance: A model study for the Prudhoe Bay reservoir, Alaska.), with measurements being performed from the surface of the ocean ice, a non-actual situation for most hydrocarbon fields in question. The relatively small gravity changes expected due to reservoir parameters as compared to the gravity variations due to noise and external variations as tides and diurnal gravity variations, requires better accuracy than has been achieved in marine geophysical surveys to date.
Existing underwater gravity meter systems are based upon lowering the gravity instrument from a ship (LaCoste, 1967; Hildebrand et al., 1990). Other systems make use of manned submersible vessels for taking measurements from within the crew compartment of the submersible vessel (Holmes and Johnson, 1993; Cochran et al, 1994, Evans, 1996; and Ballu et al., 1998). The problem of relocating the gravity meter observation point relative to the seafloor to within approximately 2 centimeters in the vertical line, is pronounced in the attempts of using a gravity meter inside a manned submersible vessel. The method is also slow in operation, and highly expensive, and running a manned submersible vessel may pose a risk to the crew. Wave action on the vessel is another noise acting on the gravity meter, and so is the noise due to inadvertent movement of the crew.
Actual Problems Implied with the Known Art
Gravity measurements taken on a surface ship or inside a submarine in motion require absolute velocity and course determination in order to perform an Exc3x6tvxc3x6s correction. Shipborne measurements of gravity are notoriosly noisy due to the ship""s accelerations from sea waves and wind, so the measurements must be low-pass filtered over long periods.
General navigation problems makes repeat measurements made by submarine or ROV uncertain with respect to position and elevation. The elevation uncertainty depends on the uncertainty of horisontal position and the local inclination of the seabed. The position and elevation problem of the known art is remedied by the present invention.
Another problem is represented by the generally unconsolidated sedimentary seabed surface. The unconsolidated sedimentary surface gives inconsistent subsidence of the gravity measurement package, either being a bottom gravity meter lowered in a cable from a ship or set out by an ROV, or measurements taken from inside a manned submarine resting on the seabed.
Drift of the gravity meter requires frequent reoccupations to the reference station. Thus the long transport time to a land-based reference station makes frequent returns to a land-based reference station unfeasible.
Use of a sea-bed reference station in a shaft near the seabed would not solve the problem with gravimeter drift of the field instrument being carried around by an ROV.
Solution to the Problem and Reference to the Claims
The above-mentioned problems are largely reduced by a method for depth measurements and monitoring of a seabed subsidence due to compaction in a hydrocarbon reservoar according to the invention defined in the attached set of claims.
The method according to the invention removes the need for making an Exc3x6tvxc3x6s correction for vessel speed and vessel course of the gravity measurements because the measurements according to the present method are made stationary at the seabed. The measurement of gravity and depth are done stationary, thus no velocity corrections are needed.
The method according to the invention using measurement stations on preinstalled benchmarks removes the measurement position uncertainty of position reoccupation to within the small area of the top surface of the benchmark, thus also removing a significant portion of the uncertainty of elevation reoccupation due to seabed inclination, by the same means.
The method according to the invention using measurement stations on preinstalled measurement stations on benchmarks reduce the problems with xe2x80x9crapidxe2x80x9d subsidence of the measurement vessel sinking in the loose sedimentary seabed due to the softness and unstability of the upper unconsolidated layers of the sediments. Heavy benchmarks which are perforated and made in concrete are preinstalled at the seabed and left to settle in the sediments for several weeks or months before a first series S1 of gravity and depth measurements. By this, two essential problems are solved:
(a) The elevation of the measurement station (with respect to the local consolidated seabed, not with regard to the earth""s gravity centre) is constant to within millimeters during a series of measurement series S1,S2, . . . , Sm taking place during several months or years.
(b) Vertical (and possibly horizontal) acceleration experienced during to slow sinking and slow settling of the measurement vessel in the local unconsolidated sediments may explain the gravity phenomenon described in Fujimoto et al., see FIG. 5 and the text at p. 910, at the bottom of the left column.
The measurement stations at preinstalled benchmarks according to the present invention prevents such sinking and settling accelerations during the measurement at each particular station.
The method according to the invention using a reference station at the seabed in the vicinity of the survey area 8 makes the return time to the reference station on the order of hours. Reoccupying a reference station onshore, as done in the known art, requires returning the gravity vessel from the seabed to the surface and making a gravity measurement either on land or at shallow depth.
All transport may incur mechanical stress that may change the instrument""s drift rate.
In the known art, the depth or pressure sensor is arranged on the submarine""s outer hull surface or other places which may not be exactly of the same relative depth with respect to the gravity sensor. According to the present invention, the depth or pressure sensor is arranged in the same elevational position with respect to the gravity sensor, the pressure sensors being arranged on the outside of the gravity sensor water-tight pressure housing. Thus the relative depth between the gravity sensor and the pressure sensors should be repeatable within far less than 1 cm, depending only on the tilt of the water-tight housing resting on the station on the benchmark.
None of the existing systems are capable of meeting the geophysical accuracy, operational speed and economical requirements of the task presented by the actual monitoring of a subsea gas reservoir, together with the need to precisely relocating the gravity instrument on the seafloor. On this background, the inventors came up with a new method and a new instrument according to the invention, solving the problems of the disadvantages of the known art.